A recent roofing job (down to the sheathing) left a filthy mess of debris in my attic blown-in cellulose insulation. I was planning to remove as many fragments of roofing trash that fell through the slats as I can, and add a layer of R-30 unfaced fiberglass across the attic floor joists.

Should I expect a decrease in R value in the old insulation from the roofing debris/dust invasion, or will it be fine once it's been fluffed up again?

You'd have a very difficult time digging debris out of loose fill cellulose. Believe me, I've lost flashlights, tape measures, etc while working in the stuff... The debris is unlikely to have much effect on the R-value, but if anything, blow on a little more cellulose and call it a day. FG batting over the top makes little sense, as does the impossible task of trying to fish out debris..